Catch a Glimpse of Vesta

For the next week there is a rare treat for the sky observer. With the unaided eye, you will be able to view an asteroid called Vesta. Vesta is the only asteroid that we can ever see with the naked eye. There are countless asteroids in our Solar System, around 50,000 have names or identifying numbers. But of all of them, only Vesta ever becomes close and bright enough for us to spot with the unaided eye. Ironically, the path that Vesta is currently taking through the constellation Virgo is very close to the path it was traveling 196 years ago when it was first discovered.

Vesta was discovered March 29, 1807 by the German physician Heinrich Olbers. Olbers thought he had found a new star but soon realized he had found the an asteroid. Olbers is best know for "Olbers Paradox," the question posing that if the cosmos if full of stars, why is the sky dark at night? But Olbers had other discoveries in addition to Vesta. He had also discovered the asteroid Pallas in 1802, he had three comets to his name as well as a clever way of computing their orbits that is still in use today.

There are tens of thousands of known asteroids, and countless other ones too small to be yet identified. They vary tremendously is type and composition. Best known are the Main Belt Asteroids. These reside in an orbit between Mars and Jupiter and have fairly circular orbits. Originally, it was thought that there had once been a planet in this orbit, and that something had caused it to be destroyed and the asteroids were the remains. We now know that there never was a planet in this orbit.

In addition to the Main Belt asteroids, many others have highly irregular orbits and some cross Earth's orbit. These are called the Apollo Asteroids. These asteroids were probably the source of the bodies which impacted the Earth, Moon and other planets during their early history. There are so far, about 1000 asteroids in the Apollo group.

Vesta is one of the brightest Main Belt asteroids and slightly over 540km in diameter but it is slightly elongated in shape. It is the second largest asteroid discovered to date. It is also quite unusual as asteroids go.

Asteroids are perhaps the oldest remnants of the early Solar System and what we learn from them tells us about our own planet's early history. They are cold, dead, and airless worlds, relatively unchanged since the first few tens of millions of years in the life of the Solar System. These bodies remain unchanged because most have not experienced the extreme temperatures, pressures and chemical alteration and crustal motion suffered by larger bodies such as planets and moons. Such forces virtually destroy records of primordial history. From clues from meteorites, we do know asteroids were heated to some degree, probably in the first few million years. They were heated enough for liquid water to permeate their interiors. Some were heated to metamorphic temperatures and some even show signs of melting but they did not reach the high temperatures the planets did.

Vesta is unique among the asteroids studied so far because it appears to have differentiated into layers like the planets. Differentiation is the separation of material according to density. This seems to imply some internal heat source. The surface layer is composed of pyroxene rich rocks, like achondrite meteorites. Underneath is an olivine mantle. Temperatures must have been unusually high (for asteroids,) to allow melting.

Hubble Space Telescope imaged Vesta in 1996 and revealed a very large, circular crater near the asteroid's south pole that is so deep it exposes the mantle. The basin is about 460 km wide and has a pronounced central peak. The floor is 12km below ground level. Since Vesta displays variations in the light reflected from its surface as it rotates, it is possible that the entire end of the asteroid was broken off revealing underlying layers which accounts for the variations in reflection. Given the size of the impact, it is likely that enough material was ejected to form a "family" of Vesta. We may well have a piece of Vesta here on Earth as a recovered meteorite from Johnstown Colorado.

NASA plans an even closer look at Vesta with a new mission called Dawn, NASA's ninth Discovery mission to be launched in May 2006. After a four year journey, the spacecraft will rendezvous with Vesta on July 30 2010, staying in orbit around the asteroid for almost an year, coming to a minimum distance of 100Km above the asteroid's surface.

It is interesting to note that although Vesta is not the largest or closest asteroid (and it is, in fact, dwarfed by Ceres 960km diameter,) it is the only one which can be spotted with the naked eye. The reason for this is that Vesta is so bright. It has a very high surface reflectivity, or albedo. Ceres reflects only about 11 percent of the sunlight striking it, which Vesta reflects a whopping 42 percent! And that is why the unaided eye can see Vesta but not Ceres.

On March 26th Vesta reached opposition to the Sun and was visible throughout the night. Opposition is when a body forms a straight line with the Earth and Sun, Earth being in the middle. For a few weeks after opposition, Vesta may be glimpsed with the unaided eye. Use the sky map to aid spotting the asteroid. Keep in mind that Vesta is not a bright object and you will need dark skies, away from artificial lights, to see it.

For those with binoculars or telescopes, Vesta will be in a favorable position through June and July. During this time, it will be highest in the sky just after dark. For those with larger telescopes, and especially those who enjoy astrophotography, Vesta will offer many great photographic opportunities as it passes by Virgo's many galaxies.